Simultaneous transverse and longitudinal mode locking in a laser employing an active absorption cell

ABSTRACT

A laser that oscillates in multiple transverse and longitudinal modes is pulsed by a mutual adaptation of an absorption cell having a substantially matching absorption line and the other components of the laser to phase-lock simultaneously the traverse and longitudinal modes of the laser by saturable absorption. Simultaneous locking is accomplished provided that the laser resonator is designed such that the longitudinal mode separation frequency is an integral multiple of the transverse mode separation frequency.

United States Patent Inventor Peter W. Smith Little Silver, N.J.

Appl. No. 737,881

Filed June 18, 1968 Patented May 25, 1971 Assignee Bell TelephoneLaboratories, Incorporated Murray Hill, NJ.

SIMULTANEOUS TRANSVERSE AND LONGITUDINAL MODE LOCKING IN A LASEREMPLOYING AN ACTIVE ABSORPTION CELL 8 Claims, 3 Drawing Figs.

US. Cl 331/94 5 Int. Cl H015 3/02 Field ofSearch 331/945; 350/160References Cited UNITED STATES PATENTS 3,321,714 5/1967 Tien 331/945D.C. EXCITATION SOURCE OTHER REFERENCES Fox et al., Mode-locked Laserand the 180 Pulse. Phys. Rev. Letters, Vol. 18, No. 20 (May 15, 1967) pp826- 828 Garmire et al., Laser Mode-locking with Saturable Absorbers.IEEE J. Quant. Elect," Vol. QE3, N0. 6 (June 1967) pp 222 -226 PrimaryExaminer-William L. Sikes Attorneys-R. J. Guenther and Arthur J.Torsiglieri the transverse mode separation frequency.

DC. EXCITATION SOURCE SIMULTANEOUS TRANSVERSE AND LONGITUDINAL MODELOCKING IN A LASER EMPLOYING AN ACTIVE ABSORPTION ClElLL BACKGROUND OFTHE INVENTION This invention relates to techniques for pulsing lasersand more particularly to techniques for simultaneously phaselocking boththe transverse and longitudinal modes of a laser.

It is sometimes remarked that lasers represent a solution in search of aproblem. Nevertheless, it is known that a laser having appropriatecharacteristics is capable of solving a particular type of problem. Forinstance, it is presently believed that lasers might be most usefullyemployed in communication systems (e.g., a PCM system) when operated ina manner that provides pulses of very narrow width that can beinterleaved in large numbers in order to reduce interpulse spacing.

One technique for providing such pulses synchronously modulating thelongitudinal modes of a laser. Synchronous modulation may be eitherphase or loss modulation at the longitudinal mode separation frequency.Such modulation causes the longitudinal modes to phase-lock and toproduce a time dependent energy distribution which is not limited in thetransverse dimension within the laser aperture,

but which, in the longitudinal dimension, is characterized by a pulse ofenergy which bounces back and forth between the resonator reflectorsproducing an output pulse every 2L/c seconds, where c is the velocity oflight and 2L is the round trip path length within the resonator.

For example, in the copending application of L. E. Hargrove, Ser. No.362,319, filed Apr. 24, 1964 (now US. Pat. No. 3,412,251 issued on Nov.19, 1968), and assigned to the assignee hereof, a technique is disclosedfor pulsing a laser by synchronously modulating the loss of theresonator with an electro-optic modulator at the longitudinal modespacing frequency c/2L. Pulsing at this rate, as previously mentioned,causes the longitudinal modes of the laser to phase-lock and produce apulse train having re petition rate c/2L. The foregoing form ofmodulation is sometimes termed loss modulation." It is also known thatnonlossy or reactive" techniques of modulation will produce laserpulsing.

It has recently been observed that lasers can pulse at the longitudinalmode separation frequency in the absence of any modulation or otherdeliberate perturbation of the laser. This pulsing has been termedself-pulsing. Most self-pulsing lasers have relatively long opticalresonators, and, according to my analysis, the long resonators permitthe population inversion of the active medium to recover betweenpassages of the pulse. Nevertheless, the observed self-pulsing of lasersis not predictable and reliable and may easily switch to a regime ofoperation in which the longitudinal modes free-run and the output poweris more or less continuous.

It has further been observed that, in a manner analogous to longitudinalmode-locking, the transverse modes of a laser phase-lock when phase orloss modulated at the transverse mode separation frequency. The timedependent energy distribution thus produced extends longitudinally theextent of the resonator but is limited transversely to a narrow region.The effect is therefore that of a spot of light which scans transverselyacross the resonator reflectors. See, for example, the copendingapplication of I. P. Kaminow-P. W. Smith, Ser. No. 728,499, filed May13, 1968, and assigned to applicants as-, signee.

A recently developed theory shows that, in a self-pulsing laser, thepulses will be approximately 11' pulses" in the laser active medium.This terminology draws upon an analogy to the magnetic resonance art,where it is already known that a pulse of oscillating magnetic field ofa certain strength and duration is effective to flip magnetic resonancedipoles in a material by exactly 180. Such a pulse is termed a 1rpulse." The analogous laser pulse, in one passage through the lasermedium, removes all available energy from the medium, leaving an excessof atoms in the lower energy level in the same amount by which there wasinitially an excess of atoms in the involves upper energy level. Theinitial upper level excess is then reestablished by the nonnal pumpingprocess prior to the next passage of the pulse through the medium.

Other types of pulsed lasers have been developed which pulse at ratessubstantially lower than the longitudinal mode spacing rate. Thesetechniques can generally be characterized as employing a bleachableabsorption cell. The more common ones employ organic dye cells as thebleachable absorption cell. Recently, similar pulsing has been obtainedin an absorption cell having discrete energy levels separated by thephoton energy of the laser radiation. Such an arrangement employing agallium arsenide injection laser and. a gallium arsenide absorption cellis disclosed by Yu. A. Drozhbin et al. Generation of ultraviolet LightPulses with a GaAs Semiconductor Laser, Soviet Physics, JETP Letters,Volume 5, page 143, Mar. 15, 1967. For many applications, the lowerpulsing rate thus obtained is inadequate.

SUMMARY OF THE INVENTION It is an object of this invention to phase locksimultaneously both the longitudinal and transverse modes of a laserreliably and without an external modulation signal at either thetransverse or longitudinal mode spacing frequency. The resultant timedependent energy distribution is limited in both the transverse andlongitudinal dimensions to a pulse of energy which zigzags back andforth between the resonator reflectors producing an output pulse eachtime it strikes one of the reflectors which is made partiallytransmissive. In addition, the output pulse scans across the reflectorand thus could be used to address an optical memory matrix.

According to the present invention, longitudinal modelocking can beproduced by using a saturable absorption cell in such a way that littleenergy is lost from the resonator to the absorption cell. Morespecifically, the laser pulses, which may be 1r pulses for the laseractive medium, could look like rrpulses or 21r pulses to the medium ofthe saturable absorption cell. By analogy with the preceding definitionof a 1r pulse, a 21: pulse is that pulse which twice fiips the relativepopulations of two energy levels and returns the absorption cell to itsinitial condition at the end of every single passage therethrough. Inaddition, it is a feature of this invention that to achieve simultaneousphase-locking of both the longitudinal and transverse modes, the cavityresonator is preferably designed such that the longitudinal modeseparation frequency is an integral multiple of the transverse modeseparation frequency. Typically, the gas absorption cell may use thesame gas as the lasing gas component of the laser active medium. Forexample, for a helium-neon laser operating at 6,328 Angstrom units, theabsorption cell may employ pure neon at a pressure of approximately 3Torr.

In one specific embodiment of my invention employing 21r pulseoperation, there is provided (1) a cavity resonator designed such thatAf,,=MAf, where Af and Af are respectively the longitudinal andtransverse mode separation frequencies and M is an integer, and ((2) amutual adaptation of the absorption cell and the other components of thelaser to provide the necessary electric field strength to provide a 271'pulse in the absorption cell. For an absorption medium having anelectric dipole moment, or oscillator strength, different from that ofthe active medium, the electric field needed for Zn pulse operationdepends inversely upon the electric dipole moment of the absorptionmedium and is adjusted accordingly. Thus, the absorption cell passes a2n pulse, as defined above. The 2w pulse flips the cell from anabsorption condition to an inverted population condition; and then thecell flips back and returns the absorbed energy coherently to the pulsebefore it leaves the cell. Thus, the cell returns to its initialabsorption condition at the end of every single pass of the pulsetherethrough. The operation of the cell in this embodiment ischaracterized by the typical time delay of self-induced transparency, asdescribed by S. L. McCall and E. L. Hahn, "Self-Induced Transparency byPulsed Coherent Light, Physical Review Letters, pages 908-911, May 22,1967. The appropriate electric field strength in the absorption cell isprovided by appropriate shaping of the beam, illustratively by focusing.The location of the active absorption cell within the laser opticalresonator in this embodiment is noncritical. Therefore, this embodimentcan optionally take the form of a ring laser.

BRIEF DESCRIPTION OF THE DRAWING The invention, together with itsobjects, features and advantages, can be easily understood from thefollowing more detailed description taken in conjunction with theaccompanying drawing, in which:

FIG. 1 is a schematic of a cavity resonator showing a typical zigzagpath of a pulse of energy in a laser in which both the longitudinal andtransverse modes are simultaneously phaselocked;

FIG. 2 is a partially pictorial and partially block diagrammaticillustration of one embodiment of the invention; and

FIG. 3 is a partially pictorial and partially block diagrammaticillustration of a modification of the embodiment of FIG. 2 employinglenses.

DESCRIPTION OF ILLUSTRA'I'IVE EMBODIMENTS Before discussing theinvention in detail, it will be helpful to describe the design of thecavity resonator employed in accordance with the principles of theinvention in order to fT= fL (2) where R1 and g2=l In such a resonator anonlinear absorbing medium, as will be more fully described hereinafter,produces a time dependent energy distribution characterized by a pulseof energy which zigzags back and forth between the reflectors. The pulseof energy thus produces an output pulse each time it is incident upon apartially transmissive reflector and due to the zigzag motiontransversely scans across the reflector. In the following discussion itwill be assumed that all embodiments employ a resonator which satisfiesequation (1 In FIG. 2 the illustrated apparatus produces laser pulsesaccording to one embodiment of the invention in which the active mediumof a laser 11 transmits pulses like those of a selfpulsing laser whilean active absorption cell responds to the pulses as 211' pulses.

The active gain medium is illustratively a mixture of helium and neon ina ratio of :1 and is capable of laser oscillation at 6,328 Angstromunits. It is contained in a suitable cylindrical tube 12 having Brewsterangle end windows 13 and 14. The active gain medium is excited, orpumped, by means of a direct-current electrical discharge betweencathode 17 and anode 18 connected across a direct-current voltage source19. The laser active medium is disposed in a linear optical resonatorcomprising reflectors 21 and 22 disposed along an axis coinciding withthe axis of tube 12. The respective curvatures of reflectors 21 and 22are adapted so that the mean diameter of the beam in the vicinity ofreflector 22, i.e., the diameter A is such as to obtain 271' pulses inthe absorption cell. The active gain medium is disposed so that itexperiences the mean diameter A An active absorption cell 31 includes agaseous medium of neon and is disposed in tube 32 to experience the meandiameter A, of the beam. Tube 32 has Brewster angle end windows 33 and34. The cell 31 also includes a cathode 37 and an anode 38 and adirect-current voltage source 39 connected between electrodes 37 and 38.It will be noted that the upper and lower levels providing theabsorption are respectively the same as the upper and lower levels ofthe lasing transition of the active medium in tube 12.

The total pressure of the active medium within the laser tube 12 isillustratively 1 Torr, the discharge length is illustrativelycentimeters and the excitation power level is illustratively 30 watts.These parameters for the laser active medium can all be varied withinthe ranges known in the laser art.

Illustratively, the pressure in tube 32 is 3 Torr. Successful operationhas been obtained at this pressure. Its length is illustratively 30centimeters. Its excitation power is illustratively 10 watts. Thespacing between end window 34 and reflector 22 is noncritical.

The typical radius of curvature of reflector 21 is 200 centimeters; andreflector 22 is illustratively substantially flat. Reflector 21 ispartially transparentto permit the extraction of a portion of the laseroscillation. The spacing between reflectors 21 and 22 is illustrativelysomewhat less than 200 centimeters.

In operation of the embodiment of FIG. 2, the presence of the saturableactive absorption cell 31 induces simultaneous phase-locking of both thelongitudinal and transverse modes supported by the resonator, the activemedium of laser 11 and its pumping means. The pumping means shouldsupply sufficient power to permit a plurality of transverse andlongitudinal modes to oscillate.

In the embodiment of FIG. 2, simultaneous phase-locking producedreliably by the cooperation of the normal components of a self-pulsinglaser, the cavity resonator design and the added active absorption cell31. The pulses obtained can be substantially shorter than those obtainedfrom a selfpulsing laser. Moreover, this pulsing is obtained without thecomplexities and power losses of a crystal modulator and its drivingcircuitry.

The absorption cell 31 presents to the laser beam a temporary absorption(loss or negative gain) which may be considerably lower than the gainprovided by the active medium of laser 11. The negative gain is providedby a larger initial population of atoms in the lower level than in theupper level. This is provided by a substantially pure neon discharge.

More specifically, the operation of the embodiment of FIG. 2 takesadvantage of beam shaping by the reflectors 21 and 22 to provide somediameter ratio (e.g., 2:1 between the portion of the beam in the laseractive medium and the portion of the beam in the active absorption cellmedium, respectively. This ratio is such that the pulse in the cavity isa 21r pulse in the absorption cell 31 and the cell absorbs very littlepower. A 27 pulse first excites, then deexcites the absorbing atoms,leaving them in the lower level. The 2w pulse passes with littleattenuation but with some characteristic delay, strikes the reflector22, is returned, and again passes through without substantialattenuation.

In absorption cell 31, the number of lower state (absorbing atoms) willbe determined by the discharge current, since in the absence of adischarge, the lower laser level, as well as the upper laser level, isnegligibly populated. This characteristic of a three-level laser mediumused as the saturable absorber should allow considerably flexibility inthe design parameters. Little loss of the laser radiation occurs in theabsorption cell 31 because its homogeneous dephasing lifetime issubstantially greater than the pulse width. It is expected that thisrelationship can be readily obtained in gases and other absorbing mediahaving inhomogeneously broadened absorption lines. Inhomogeneousbroadening is a broadening of the gain curve or absorption loss curve,in this case, such that depleting the gain at one frequency does notsubstantially deplete the available absorption at other frequenciesrelatively widely separated from the one frequency within the broadenedcurve. Nevertheless, the invention is not limited to inhomogeneouslybroadened absorption media.

Exactly the same mode of operation can be achieved in an opticalresonator having reflectors 41 and 42 that are both substantially flat,as shown in the modified embodiment of FIG. 3. ln FIG. 3, the relativediameter ratio of the beam in the laser 11 and the absorption cell 31can be achieved by lenses 61 and 62 disposed within the opticalresonator along the axis thereof. The lenses 61 and 62 areillustratively disposed between the laser 11 and the active absorptioncell 31 and provide essentially parallel beams through both the laser 11and the absorption cell 31. A single lens in any suitable position inthe optical resonator will sufi'ice if one wishes to permit a moredivergent beam in at least one of the two tubes 12 and 32.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

I claim:

1. ln a laser which simultaneously oscillates in a plurality oftransverse and a plurality N of longitudinal modes and which includes anactive gain medium capable of stimulated emission of coherent radiation,means for pumping said active medium, and a cavity resonator of length Lincluding said medium and supporting said modes; the improvement,comprising:

means for constraining the radiation within said resonator so that thelongitudinal mode separation frequency Af is an integral multiple M ofthe transverse mode separation frequency Af and an absorption celldisposed within said resonator, said cell comprising an absorbing mediumhaving energy levels at least two of which are separated byapproximately the photon energy of the coherent radiation of said activegain medium,

whereby said absorption cell induces simultaneous phaselocking of boththe longitudinal and transverse modes supported by said resonator.

2. The optical apparatus of claim 1 wherein said resonator comprisesconcave reflectors of radii R and R and satisfying the relationship 3.The optical apparatus of claim 1 in combination with means for providingin said absorbing medium a population of the lower of the two levelsgreater than the population of the upper of the two levels by an amountstill permitting laser oscillation, said cell being disposed andmutually adapted with respect to the other components of said laser toabsorb energy from a pulse of said radiation and then return the energyto said pulse at least once for every two passes of said pulse throughsaid cell.

4. Optical apparatus according to claim 1 in which electric fieldstrengths and electric dipole moments in the absorption medium aremutually adjusted for 27T pulse operation.

5. Optical apparatus according to claim 1 in which the mutual adaptationof the absorption cell and the other components of the laser comprises agaseous absorbing medium, said absorbing medium having a pressure equalto or less than a value providing a homogeneous dephasing lifetimegreater than the pulse width.

6. Optical apparatus according to claim 5 in which the mutual adaptationof the cell and the other components of the laser include means withinthe laserfor providing an electric field strength of the radiation insaid active absorption cell medium that is adjusted relative to thefield strength of said radiation in the laser active medium and theelectric dipole moments of said media for 211' pulse operation, wherebythe absorption cell absorbs energy from said radiation and then returnsthe absorbed energy to said radiation during every single pass of saidradiation through said absorption cell.

7. Optical apparatus according to claim 6 in which the electric fieldstrength providing means comprises at least one lens disposed betweenthe laser active gain medium and the active absorption cell medium tofocus the radiation in said active absorption cell medium to across-sectional diameter sufficient to obtain 21r pulse operation.

8. Optical apparatus according to claim 6 in which the electric fieldstrength providing means includes an adaptation of the resonating meansin which focusing reflectors produce a beam of said radiation having afirst diameter in a first locality and a second diameter in a secondlocality, the electric field strength providing means further comprisingdisposition of the laser active gain medium in the first locality andthe active absorption cell medium in the second locality.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3: 5228 Dated May 25, 1971 Inventor(s) Peter W Smith It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 56, change "Af MAf" to --Af M/Af L L T Column 3, line 33,change "AfL" to --Af Column 3, equation (1), change "LII MA to "Af MlxfColumn 3, equation 2, change "cos-l" to --cos Column 4, line 66, change"considerably" to --consider'able-.

Column 6, line 1, change 'cos-l" to --cos' Signed and sealed this 13thday of June 1972.

(SEAL) Attest:

EDWARD M.FLE'ICHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM uscoMM-oc (scam-pas U S. GOVERNMENY PRNTING OFFICE 1 9.90356'334

2. The optical apparatus of claim 1 wherein said resonator comprisesconcave reflectors of radii R1 and R2 and satisfying the relationship 3.The optical apparatus of claim 1 in combination with means for providingin said absorbing medium a population of the lower of the two levelsgreater than the population of the upper of the two levels by an amountstill permitting laser oscillation, said cell being disposed andmutually adapted with respect to the other components of said laser toabsorb energy from a pulse of said radiation and then return the energyto said pulse at least once for every two passes of said pulse throughsaid cell.
 4. Optical apparatus according to claim 1 in which electricfield strengths and electric dipole moments in the absorption medium aremutually adjusted for 2 pi pulse operation.
 5. Optical apparatusaccording to claim 1 in which the mutual adaptation of the absorptioncell and the other components of the laser comprises a gaseous absorbingmedium, said absorbing medium having a pressure equal to or less than avalue providing a homogeneous dephasing lifetime greater than the pulsewidth.
 6. Optical apparatus according to claim 5 in which the mutualadaptation of the cell and the other components of the laser includemeans within the laser for providing an electric field strength of theradiation in said active absorption cell medium that is adjustedrelative to the field strength of said radiation in the laser activemedium and the electric dipole moments of said media for 2 pi pulseoperation, whereby the absorption cell absorbs energy from saidradiation and then returns the absorbed energy to said radiation duringevery single pass of said radiation through said absorption cell. 7.Optical apparatus according to claim 6 in which the electric fieldstrength providing means comprises at least one lens disposed betweenthe laser active gain medium and the active absorption cell medium tofocus the radiation in said active absorption cell medium to across-sectional diameter sufficient to obtain 2 pi pulse operation. 8.Optical apparatus according to claim 6 in which the electric fieldstrength providing means includes an adaptation of the resonating meansin which focusing reflectors produce a beam of said radiation having afirst diameter in a first locality and a second diameter in a secondlocality, the electric field strength providing means further comprisingdisposition of the laser active gain medium in the first locality andthe active absorption cell medium in the second locality.